Instrument shunts



Oct. 16, 1962 U. L. SMITH INSTRUMENT SHUNTS Filed Aug. 7. 1959 WITNESSES Fig. 4.

Load

Fig.7.

\NVENTOR Uel L.Smith.

id/W ATTORNEY United States Patent 3,359,182 INSTRUMENT SHUNTS Uel L. Smith, Morristown, N .J assignor t0 Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 7, 1959, Ser. No. 832,365 Claims. (Cl. 324-126) This invention relates to instrument shunts, and it has particular relation to external shunts used in conjunction with direct-current measuring instruments.

Shunts are used in the measurement of relatively large direct currents where it is not practical to carry the full current of a circuit through the instrument. Such shunts generally are used with a standard indicating or recording instrument having a 50 or 100 millivolt movement. The instrument may be either of .the portable or of the switchboard-mounted type. For the former a portable shunt may be employed, and for the latter a switchboard type shunt may be used which is designed for permanent installation in bus bar circuits.

One type of external instrument shunt in common use includes a pair of relativelymassive terminal castings, preferably of copper, and one or more resistance strips or shunt elements, the terminal castings having slots milled therein for reception of the shunt elements. In addition, depending on the shunts current capacity, each of the terminal castings may be provided with one or more blades for connection to circuit bus bars. Where such blades are furnished, they are either formed by additional milling of the terminal castings or inserted into blade slots milled in the castings. The shunt ele ments and the blades, where the latter are inserted into blade slots, are secured to the terminal castings as by soldering. The terminal castings and the blades are provided with suitable means such as apertures or screw-type terminals for making external circuit connections to the shunt. For higher current-carrying capacities, larger terminal castings are provided with a greater number of milled slots for the reception of additional shunt elements and additional bus bar connecting blades.

In a shunt of the foregoing construction where multiple bus bar connecting blades are provided, the currents in the bus bar connecting blades may be unequal. In such instance, the reading of the millivoltmeter used with the shunt will be in error. To make this error as small as possible, the cross sections of the terminals are made large. This has resulted in shunts of massive size and rigid construction. I

In accordance with the invention, the terminals of an instrument shunt are fabricated from standard commercial electrical conducting material to produce a relatively lightweight unit. Conveniently, such material may take the form of standard bus bar copper. One or two shunt elements or strips are secured directly to each of two relatively short sections of such material. For larger capacities, a plurality of subassemblies of the foregoing type may be stacked and secured to additional bus bar conductors, thus forming a high-current multiple unit. It will be appreciated that an instrument shunt constructed in accordance with the invention obviates the necessity of casting the terminals and milling slots therein, thus reducing the cost of the shunt as a result of savings in both labor and material. As a consequence of the reduction in the amount of copper used for the shunt terminals, a shunt of lower overall weight is produced. In addition, shunts of the type disclosed herein are more flexibly installed in service.

According to a further aspect of the invention, the inaccuracies in meter readings caused by unequal currents in the bus bar conductors of the subassemblies of 3,059,182 Patented Oct. 16, 1962 a multiple shunt are eliminated. For this purpose, relatively low-value bridging impedances, preferably resistors, are connected between the subassemblies, and a meter connection is made to each of two groups of these impedances.

It is, therefore, an object of the invention to'provide an improved instrument shunt.

It is a further object of the invention to provide an instrument shunt whose terminals are fabricated of standard commercial electrical conducting material.

It is another object of the invention to provide an instrument shunt as defined in the preceding object, wherein the conducting material is standard bus bar copper.

It is an additional object of the invention to provide an instrument shunt as defined in either of the two preceding objects, wherein a shunt element is secured directly to corresponding surfaces of each of a pair of such terminals.

It is still another object of the invention to provide an instrument shunt which is constructed of a plurality of spaced, stacked shunt subassemblies, each having a struc ture similar to that set forth in the preceding object.

It is still a further object of the invention to provide a multiple instrument shunt having improved means for eliminating meter reading errors caused by unequal bus currents.

It is also an object of the invention to provide an instrument shunt as defined in the preceding object wherein said means comprise a plurality of bridging impedances connected between shunt subassemblies.

Other objects of the invention will be apparent from the following discussion taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a view in elevation of an instrument shunt assembly embodying the invention; v

FIG. 2 is a top plan view of the instrument shunt assembly of FIG. 1;

FIG. 3 is a view in section taken along the lines III-III ofFIG. 1;

FIG. 4 isa view in elevation, with portions broken away, of a bus-bar-mounted instrument shunt constructed of a plurality of shunt subassemblies;

FIG. 5 is a schematic view of a multiple instrument shunt employing bridging resistors;

FIG. 6 is afragmentary view in elevation, with parts omitted, of a plurality of instrument shunt subassemblies secured directly to a multiple-conductor bus bar; and

FIG. 7 is a schematic view illustrating an electrical circuit which incorporates an instrument shunt embodying the invention.

The instrument shunt assembly ofv FIGS. 1, 2 and 3 includes a pair of flat terminal plates 1 and a pair of flat shunt elements or strips 3. The terminal plates 1 conveniently may be fabricated of standard commercial bus bar copper which may have a thickness dimension, for example, of approximately inch. Each of the terminal plates has an end surface 5 which is spaced from the corresponding end surface of the other terminal plate. The shunt elements 3 are overlappingly secured as by soldering or welding to opposite surfaces 7 and 9 ofthe terminal plates 1, such that the elements are separated by the thickness dimension of the terminal plates.

The shunt strips 3 may be fabricated of any suitable resistance material such as a copper-nickel-manganese alloy which, is commercially available under the name of Manganin. This material has a negligible temperature coeflicient and thermoelectric effect so that errors resulting from heating. of the shunt elements by the currents which they carry are. avoided. It will be understood that such heat as is evolved will be dissipated by conduction as well as by convection and radiation due to the construction of the shunt assembly.

Each of the bus bar terminal plates 1 may be provided with a screw-type terminal 11 for securing instrument leads to the shunt assembly. In addition, each terminal plate is provided with an aperture 13 for securing the shunt assembly to the bus bars of the circuit with which it is associated.

It should be noted that the terminal plates 1 and the shunt strips 3 need not necessarily be of the same Width. For example, in certain applications it may be desirable that the terminal conductors be wider than the shunt strips.

Referring now to FIG. 4, a shunt assembly is illustrated which comprises three parallel shunt subassemblies 15, each constructed substantially in accordance with the shunt assembly of FIG. 1. The shunt subassemblies 15 are aligned in spaced, stacked relationship. Interleaved with the terminal plates of adjacent subassemblies 15 are terminal members 17. The terminal members 17 also may be fabricated of standard commercial bus bar copper of the same thickness as that of the terminal plates of the subassemblies. The terminal plates of the subassemblies are secured to the terminal members 17 as by soldering or welding.

It will be appreciated that in order to prevent interference between adjacent subassemblies 15, the overall height of each subassembly (h, FIG. 3) must not exceed twice the thickness dimension of each of the terminal members 17. For instance, if the terminal members 17 are constructed of bus bar material having a A-inch thickness dimension, the dimension h must not exceed /z-inch.

The terminal members 17 are interleaved with parallel bus bars 21 of the bus with which the shunt assembly is associated. Each of the bus bars 21 has a thickness dimension which is equal to the thickness dimension of each of the terminal members 17 and of each of the terminal plates of the shunt subassemblies 15; and the spacing between adjacent bus bars 21 also is equal to said thickness dimension. Each of the terminal members 17 is provided with an aperture 25, while each of the bus bars 21 is provided with an aperture 27. Adjacent apertures 25 and 27 are aligned to permit reception of bolts 29, the bolts 29 being fastened securely by means of nuts 31.

It will be observed that if the shunt assembly of FIG. 1 has a drop of 50 millivolts, for example, when carrying its rated current, the shunt assembly of FIG. 4 may have a rating of three times the current rating of the shunt assembly of FIG. 1 to obtain a 50 millivolt drop. This follows from the fact that each of the shunt subassemblies 15 may have a current rating which is the same as the current rating of the shunt assembly of FIG. 1. Further variations in shunt current-carrying capacities and millivolt ratings may be obtained by varying the sizes of the shunt elements, the terminal plates 1 and the terminal members 17. It will be appreciated that as many shunt subassemblies as are required may be paralleled to form a multiple unit of the type illustrated in FIG. 4. As was pointed out heretofore, elimination of the casting and the milling of shunt terminals by employing the teachings of the invention greatly reduces the cost and overall weight of an instrument shunt.

Terminal screws 32 similar to the terminal screws 11 of FIG. 1 may be provided for securing instrument terminal leads to the shunt assembly of FIG. 4. It will be recalled, however, that currents in the shunt subassemblies of FIG. 4 may be unequal, as a result of which the reading of a millivoltmeter used with the shunt will be in error. To eliminate this error, the shunt assembly of FIG. 4 may be provided with a plurality of bridging resistors 33. One of the shunt elements associated with each shunt subassembly 15 is provided with a pair of such resistors. As is shown more clearly in FIGS. 2 and 3, one end of each of the resistors 33 is secured to one of the shunt elements 3 as by welding or soldering. Alternatively, the bridging resistors may be secured to the terminal plates 1 adjacent opposite ends of the shunt element as by bolting. Each of the bridging resistors '33 is of relatively low value, and for a given shunt assembly comprising a plurality of shunt subassemblies as illustrated in FIG. 4, all of the bridging resistors associated therewith are of equal resistance. In practice, each of these resistors preferably is a rigid or self-supporting piece of wire or strip of a material such as copper, brass or Manganin. The other end of each resistor is brought out to a. position wherein it projects beyond the side of its associated shunt element. The last-named end of each resistor may be provided with an eyelet terminal 34. The terminals of corresponding bridging resistors of the shunt subassemblies 15 are connected together rigidly by means'of wire 35. Alternatively, spacers may be inserted between adjacent terminals 34, and a bolt may be run through each group of terminals and their associated spacers.

A meter to be used with the shunt assembly of FIG. 4 may be connected by any suitable means to each of the common terminal connections of the two groups of resistors. Thus, millivoltmeter connections are made to the exact centers of the bridging resistors, as is shown schematically in FIG. 5 for a pair of adjacent shunt subassemblies at points 330. It can be shown that the millivolt reading taken at these points is always correct, regardless of the division of bus current in the subassemblies; and it also can be shown that as many subassemblies as are desired may be paralleled as long as all of the bridging resistors are of equal resistance. The exact value of this resistance is unimportant as long as it is relatively low and as long as all of the resistors are of equal value.

If bridging resistors as aforementioned are used, the terminal screws 32 may be eliminated. Conversely, if but a single unit, such as is illustrated in FIGS. 1, 2 and 3, is used, the bridging resistors 33 may be eliminated, since the problem. of unequal currents then is non-existent, and the meter connections may be made to the screwtype terminals 11.

It will be understood that the shunt subassemblies 15 of FIG. 4 need not necessarily be secured to the terminal members 17 in order to obtain a shunt assembly of relatively high current-carrying capacity. As is shown in FIG. 6, for example, a plurality of shunt subassemblies, such as that illustrated in FIG. 1, may be secured in parallel in the field directly to a bus having multiple bus bar conductors 34 by means of the apertures 16. Thus, a series of standard units may be designed in accordance with FIG. 1 which have ratings of 100, 500 and 2,000 amperes, respectively. For a 10,000-ampere shunt, for example, five LOGO-ampere units may be connected in parallel directly to a multiple-conductor bus.

The circuit of FIG. 7 comprises a direct-current source 35 suitably connected by means of buses, indicated by the numerals 36, 37 and 39, in series circuit relationship with a load 41 and a multiple instrument shunt 43, which is constructed in accordance with the embodiment of the invention illustrated in FIG. 6. The buses 37 and 39 are connected to the shunt 43 by means of bolts 45 and 47, respectively. An instrument such as a direct-current millivolt-meter 49 is connected by any suitable means to bridging resistors 51 of the shunt 43 by means of instrument leads 53. Thus, since the voltage drop across the shunt 43 varies linearly with the current therethrough, the instrument 49 may be calibrated to read such current directly.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications falling within the spirit and scope of the invention are possible.

I claim as my invention:

1. A shunt assembly comprising a plurality of shunt subassemblies, each of said subassemblies including a pair of spaced, complete, electroconductive plates having opposed end surfaces, each of said subassemblies further including a shunt element electroconductively secured to a side surface of each of said plates adjacent said opposed end surfaces, respectively, to bridge the space between said plates, and means mounting said subassemblies in spaced, stacked relationship, said means comprising a plurality of spaced electroconductive members each in interleaving engagement with corresponding plates of two immediately adjacent subassemblies and extending beyond the ends of the last-named subassemblies to constitute a terminal for connecting said last-named subassemblies in an electrical circuit.

2. A shunt assembly comprising a plurality of like shunt subassemblies, each of said subassemblies including a separate pair of like, spaced, complete, electroconductive terminal plates having opposed end surfaces, each of said plates having two parallel faces interconnected by itsassociated end surface, each of said subassemblies further including a separate pair of like, spaced, substantially parallel shunt strips, each of said shunt strips being overlappingly and electroconductively secured to corresponding respective ones of said faces of its associated terminal plates to bridge the space therebetween with said opposed end surfaces of each pair of terminal plates being interposed between the pair of shunt strips associated therewith, means mounting said subassemblies in spaced, stacked relationship, said subassemblies being parallel to and aligned with each other in parallel planes, the spacing between corresponding terminal plates of immediately adjacent ones of said subassemblies being equal to the spacing between the parallel faces of each of said terminal plates.

3. A shunt assembly comprising a plurality of like shunt subassemblies, each of said subassemblies including a separate pair of like, spaced, complete, electroconductive terminal plates'having opposed end surfaces, each of said plates having parallel first and second spaced faces interconnected by its associated end surface, each of said subassemblies further including separate first and second shunt strips overlappingly and electroconductively secured to said first and second faces, respectively, of their associated terminal plates, said opposed end surfaces of each pair of terminal plates being interposed between the first and second shunt strips associated therewith, and means mounting said subassemblies in spaced, stacked relationship, said subassemblies being parallel to and aligned with each other in parallel planes, the spacing between corresponding terminal plates of immediately adjacent ones of said subassemblies being equal to the spacing between the parallel faces of each of said terminal plates, said spacing having an upper limit of onequarter inch, all portions of said terminal plates and said shunt strips of each subassembly being located between two parallel planes which are spaced by one-half inch.

4. A shunt assembly for installation in a current-carrying bus having a plurality of similar, parallel, stacked bus bar conductors of equal thickness dimension, in which immediately adjacent ones of said bus bar conductors are spaced from each other by a distance equal to said thickness dimension, said shunt assembly comprising a plurality of shunt subassemblies, each of said subassemblies including a separate pair of spaced, in-line bus bar terminal plates of said thickness dimension and a separate pair of spaced parallel shunt strips, each of said shunt strips being overlappingly and electroconductively secured to corresponding surfaces of their associated pair of terminal plates, said shunt strips being spaced in a direction parallel to said thickness dimension, a plurality of bus bar terminal members of said thickness dimension mounting said subassemblies in spaced, stacked relationship, said subassemblies being parallel to and aligned with each other in parallel planes, each of said terminal members interleaving with corresponding terminal plates of two immediately adjacent ones of said stacked subassemblies, and means electroconductively securing interleaved terminal plates and terminal members to each other, said terminal members extending beyond opposite ends of said subassemblies for insertion in the spaces between said bus bar conductors.

5. A shunt assembly comprising a plurality of like, spaced shunt subassemblies, each of said subassemblies including a pair of spaced, complete, electroconductive terminal plates having opposed end surfaces and a shunt element electroconductively secured to a side surface of each of said plates adjacent said opposed end surfaces, respectively, to bridge the space between said plates, and a pair of electrical impedance elements spaced in the direction of spacing of said terminal plates for each subassembly, all of said impedance elements being of equal impedance, each of said impedance elements having a first and a second terminal end, means electroconductively securing the first terminal ends of corresponding impedance elements of said subassemblies to corresponding points of their respective associated subassemblies, at least a part of the shunt element of each subassembly being interposed between the first terminal ends of its associated pair of impedance elements, and means electroconductively securing the second terminal ends of corresponding impedance elements of said subassemblies to each other to provide a pair of terminals for making external instrument connections to said shunt assembly.

6. A shunt assembly comprising a plurality of like spaced shunt subassemblies, each of said subassemblies including a separate pair of spaced, complete, electroconductive terminal plates having opposed end surfacesand a separate pair of spaced shunt elements, each of said terminal plates having two parallel faces interconnected by its associated end surface, each of said shunt elements of each subassembly being electroconductively secured to corresponding respective ones of said faces of their associated terminal plates with the associated end surfaces thereof being interposed between the shunt elements associated therewith, and a pair of electrical resistance elements spaced in the direction of spacing of said terminal plates for each subassembly, all of said resistance elements being of equal resistance, each of said resistance elements having a first and a second terminal end, means electroconductively securing the first terminal ends of corresponding resistance elements of said subassemblies to corresponding points of their respective associated subassemblies, at least a part of one shunt element of each subassembly being interposed between the first terminal ends of its associated pair of resistance elements, and means electroconductively securing the second terminal ends of corresponding resistance elements of said subassemblies to each other to provide a pair of terminals for making external instrument connections to said shunt assembly.

7. An instrument shunt unit for installation in a single electrical current-carrying bus comprising a plurality of like, stacked, uniformly-spaced bus bar conductors parallel to and aligned with each other in spaced parallel planes, said shunt unit comprising a plurality of like, uniformly-spaced shunt subassemblies and terminal means for interleaving electroconductive engagement with said bus bar conductors, each of said subassemblies including a shunt element for conducting therethrough a portion of the current flowing in said bus, and a pair of self-supporting bridging resistors for each subassembly spaced in the direction of current flow in said bus, all of said bridging resistors being of equal resistance, each of said bridging resistors having a first and a second terminal end, means electrtoconductively securing the first terminal ends of corresponding bridging resistors of said subassemblies to corresponding points of their respective associated subassemblies, at least a part of the shunt element of each subassembly being interposed between the first terminal ends of its associated pair of bridging resistors, and means electroconductively and rigidly securing the second terminal ends of corresponding bridging resistors of said subassemblies to each other to provide a pair of terminals for making external instrument connections to said shunt unit.

8. A shunt assembly comprising a plurality of like shunt subassemblies, each of said subassemblies including a separate pair of like, spaced, complete, electroconductive terminal plates having opposed end surfaces, each of said plates having parallel first and second spaced faces interconnected by its associated end surface, each of said subassemblies further including separate first and second like shunt strips overlappingly and electroconductively secured to said first and second faces, respectively, of their associated terminal plates, said opposed end-surfaces of each pair of terminal plates being interposed between the first and second shunt strips associated therewith, means mounting said subassemblies .in spaced, stacked relationship, said subassemblies being parallel to and aligned with each other in parallel planes, the spacing between corresponding terminal plates of immediately adjacent ones of said subassemblies being equal to the spacing between the parallel faces of each of said terminal plates, and first and second self-supporting bridging resistors for each subassembly spaced in the direction of spacing of said terminal plates, all of said bridging resistors being of equal resistance, each of said bridging resistors having a first and a second terminal end, means electroconductively securing the first terminal ends of said first resistors to first corresponding points of the first shunt strips of their respective associated subassemblies, means electroconductively securing the first terminal ends of said second resistors to second corresponding points of the first shunt strips of their respective associated subassemblies, at least a part of the first shunt strip of each subassembly being interposed between the first terminal ends of its associated first and second resistors, means electroconductively and rigidly securing the second terminal ends of said first resistors to each other to provide a first terminal for making an external instrument connection to said shunt assembly, and means electroconductively and rigidly securing the second terminal ends of said second resistors to each other to provide a second terminal for 8 making an external instrument connection to said shunt assembly.

9. A shunt assembly comprising a pair of spaced, complete, electroconductive terminal plates having opposed end surfaces, a shunt element electroconductively secured to a side surface of each of said plates adjacent said end surfaces, respectively, to bridge the space between said plates, and a pair of resistors of equal value spaced in the direction of spacing of said terminal plates, each of said resistors having a first and a second terminal end, and means electroconductively securing the first terminal ends of said resistors to said assembly, at least a part of said shunt element being interposed between the first terminal ends of said resistors.

10. A shunt assembly comprising a pair of spaced, inline, like, rectangular, terminal plates each formed completely from a length of standard electroconductive bus bar having a thickness dimension of one-quarter inch, a pair of like, spaced, substantially parallel shunt strips overlappingly and electroconductively secured to faces of said terminal plates which are separated by the respective thickness dimensions thereof to bridge the space between said terminal plates, and a pair of self-supporting resistors of equal value spaced in the direction of spacing of said terminal plates, each of said resistors having a first and a second terminal end, and means electroconductively securing the first terminal ends of said resistors to said assembly, at least a part of one of said shunt strips being interposed between the first terminal ends of said resistors, all portions of said terminal plates, shunt strips and resistors being located between two parallel planes spaced by one-half inch.

References Cited in the file of this patent UNITED STATES PATENTS 869,796 Mug Oct. 29, 1907 1,084,721 Willis Jan. 20, 1914 1,807,852 MacGahan June 2, 1931 FOREIGN PATENTS 19,993 Great Britain 1904 

